A circulator is an electrical device made using a ferrite loaded symmetrical junction of three or more regularly spaced transmission lines, which device has nonreciprocal operation, preferring progression of electromagnetic fields in one circular direction. Thus, during operation, a circulator has a property of transferring power from its so-called incident port to the next adjacent port and isolating all other ports. Properties that characterize circulator performance include insertion loss, return loss, and isolation (insertion loss in the undesired direction) and band width (frequency range of operation).
FIG. 1A is a functional diagram of a prior art, three-port circulator 100 (also referred to herein as a Y-junction circulator), which is unique, passive, non-reciprocal symmetrical junction device having one typical input port, one output port, and one decoupled port, in which a microwave or radio frequency signal entering any port is transmitted to the next port in rotation (only). The circulator 100 of FIG. 1A provides transmission of energy from one of its ports to an adjacent port, while decoupling the signal from all other ports. The circulator symbol shown in FIG. 1A, for example indicates that the RF energy incident on port 1 emerges from port 2, entering port 2 also be used as an isolator or a switch, and is simple in construction, compact, and, in at least some applications, lightweight. Circulators can be implemented using resonant structures such as radio frequency resonant cavities and in waveguide at higher frequencies. Circulators may also be realized in planar configuration using stripline or microstrip technology which employ a planar resonating element between two ground plane conductors (stripline) or coupled to a single ground plane conductor (microstrip). Examples of microstrip and stripline circulator construction are provided, for example, in U.S. Pat. No. 4,704,588, which is hereby incorporated by reference. Additional examples of stripline circulator construction are provided, for example, in U.S. Pat. No. 3,758,878, which is hereby incorporated by reference.
Additional types of circulators include isolators (a three-port circulator with one port terminated in a matched load) and duplexers (four-port circulators, often used in radar systems and to separate received and transmitted signals in a transmitter). A related type of electrical device is an isolator, which is a two-port device that transmits microwave or radio frequency power in one direction only. Isolators can be used to shield a circuit on its input side, from the effects of conditions on its output side (e.g., an isolator can help prevent a microwave source being detuned by a mismatched load.) A three port circulator can be turned into an isolator by terminating one of its three ports with a matched load.
RF circulators further can divide into the subcategories of 3 or 4-port waveguide circulators based on Faraday rotation of waves propagating in a magnetized material, and 3-port “Y-junction” circulators based on cancellation of waves propagating over two different paths near a magnetized material. The Y-junction circulator can be constructed in either rectangular waveguide or stripline. Waveguide circulators may be of either 4-port or 3-port type, while more compact devices based on striplines generally are of the 3-port type, and are generally used with high microwave frequencies. Stripline circulators are generally used with VHF and low microwave frequencies and often are made using coaxial connectors. In both types of circulators, a ferrite element is placed in the center of three symmetrical junctions that are spaced 120 degrees apart. A ferrite post is used in the waveguide circulator, and two ferrite disks, one located on each side of a metal center conductor, are used in the stripline circulator.
Ferrite stripline circulators also can be referred to in the art as ferrite stripline junction circulators. A stripline junction circulator is a three-port non-reciprocal microwave junction used to connect a single antenna to both a transmitter and a receiver. For example, FIG. 1B is a schematic diagram of a prior art, three port stripline circulator 105. This exemplary three port ferrite stripline circulator 105 of FIG. 1B is made using two planar ferrite disk resonators 120a, 120b, symmetrically coupled by three transmission lines 130a, 130b, 130c (sometimes referred to as “resonating elements”), formed into a “Y” shape, where the ferrite disks 120a, 120b, and the intersection of the 3 transmission lines 130a, 130b, 130c from the Y-junction is where the actual circulation occurs. The two ferrite disc resonators 120a, 120b are spaced between a conducting center plate (e.g., the center conductors 130) and two conducting ground planes (110a, 110b), and two permanent magnets 112a, 112b, which provide a magnetic bias to the ferrite disc resonators 120a, 120b, respectively.
The magnetic bias from the permanent magnets 112a, 112b helps to achieve power flow in the preferred direction(s). The static biasing magnetic field 140 from permanent magnets 112a, 112b is oriented perpendicular to the plane in which the junction of transmission lines 130as, 130b, 130c lie, as shown in FIG. 1B. Each of the permanent magnets 112a, 112b behaves like a respective magnetic pole that helps to orient the magnetic field.
Depending upon particular requirements of the circulator 105, a high permeability spacer (not shown) may be used to focus or spread the magnetic field 140. In addition, as will be understood in the art, one or both of the permanent magnets 112a, 112b may include a pole piece. A pole piece attaches to and in a sense extends a pole of the magnet 112. A pole piece (which is not shown in FIG. 1B), is a structure that attaches to the magnet and helps to extend the pole of the magnet by directing the magnetic field produced by a magnet. The pole piece usually is made of high magnetic permeability material.
With ferrite resonator-based circulators, the nonreciprocal characteristics of the ferrite resonator 120, under the influence of proper magnetic bias fields (from the permanent magnets 112), make the aforementioned power transfer possible. One permanent magnet (in a microstrip circulator) or two (in a stripline circulator) provides the required magnetic field to induce the non-reciprocal behavior of the ferrite (gyromagnetic).
Ferrites can be divided into two families based on their magnetic coercivity (their resistance to being demagnetized): hard ferrites (difficult to degmagnetize) and soft ferrites (easy to demagnetize). Circulators typically use soft ferrites and, thus, many circulators require a separate bias magnet (e.g., magnet 112) to apply a bias to the ferrite. This can add bulk and weight to the circulator.
Although FIG. 1B illustrates a prior art stripline circulator, one of skill in the art will appreciate that a microstrip circulator includes some similar components, but instead of having its transmission lines 130a-c (which also are collectively referred to as a planar resonating element) disposed between two ground plane conductors 110a, 110b, two ferrite disks 120a, 120b, and two biasing magnet 112a, 112bs, in a microstrip circulator, the transmission lines 130a-c can instead be coupled to a single ground plane conductor (microstrip), using a single ferrite biased by a single biasing magnet. Also, although not shown, one will appreciate that at least some prior art circulators are contained in a high permeability housing, which also directs the field of the biasing magnet(s) used.
Referring still to the stripline circulator 105 of FIG. 1B, when one of the ports 130a, 130b, 130c of the stripline circulator 105 is appropriately terminated, with either an internal or external termination, the stripline circulator 105 then becomes an isolator which isolates the incident and reflected signals. Thus, a signal applied to the ferrite disk pair 120a, 120b, will generate two equal, circularly polarized counter-rotating waves (similar to the arrows shown in FIG. 1A) that will rotate at velocities ω+ and ω−. The velocity of a circularly polarized wave as it propagates through a magnetically biased microwave ferrite material depends on its direction of rotation. By selecting the proper ferrite material and biasing magnetic field, the phase velocity of the wave traveling in one direction can be made greater than the wave traveling in the opposite direction.
For example, referring to FIGS. 1A and 1B, if a signal were applied at Port 1 (e.g., transmission line 130a); the two waves will arrive in phase at Port 2 (e.g., transmission line 130b) and cancel at Port 3 (e.g., transmission line 130c). Maximum power transfer will occur from Port 1 to 2 and minimum transfer from Port 1 to 3, depending on the direction of the applied magnetic field. Due to the symmetry of the Y-Junction, similar results can be obtained for other port combinations. Externally the circulator seem to direct the signal flow clockwise or counterclockwise depending on the polarization of the magnetic biasing field.
FIG. 1C is a schematic diagram of a prior art, three port waveguide circulator 115. Although FIG. 1C shows the waveguide circulator 115 having three H-plane junctions, Electric field-plane (E-plane) circulators can also be made (for clarity, the magnet 112 is not shown in FIG. 1C). Operation in the circulator 115 of FIG. 1C is generally similar to that of FIG. 1B.